The present invention relates to a support structure of sensors for detecting the number of rotation of a bevel gear. More specifically, the present invention relates to a sensor support structure for detecting the number of the rotation of a ring gear of a differential apparatus provided in vehicles, based on detecting the teeth of the ring gear.
FIG. 3 shows part of a prior art differential apparatus 20. As shown in the figure, the differential apparatus 20 has an outer case 23 and an inner case 27. The inner case 27 is rotatably supported in the outer case 23. A ring gear 21, which is a bevel gear, is fixed to the inner case 27. The ring gear 21 is meshed with a drive pinion (not shown) fixed to the distal end of a drive shaft (not shown). When the drive shaft rotates, the ring gear 21 rotates with the inner case 27. The rotation of the ring gear 21 is transferred to right and left axles 30 (only one is shown) by differential gears 28, 29 in the inner case 27.
A magnetic sensor 22 is installed in the differential apparatus 20 to detect the rotation speed of the ring gear 21. The magnetic sensor 22 is inserted in an installation hole 24 provided in the outer case 23. The magnetic sensor 22 is fixed to the outer case 23, with a positioning flange 22a in contact with a positioning surface 24a of the hole 24.
The magnetic sensor 22 includes a detection surface 25 facing the edge 26a of each tooth 26 of the ring gear 21. When the ring gear 21 rotates, the magnetic sensor 22 detects the teeth that pass by the detection surface 25. Accordingly, the rotation speed of the ring gear 21 is obtained based on the detected number of the teeth 26 per unit time. The speed of the ring gear 21 may be used to determine the vehicle speed.
The magnetic sensor 22 includes a magnet and a magnetic detection element such as an electromagnetic pickup coil, a hall element, and a magnetic resistance element. The magnetic flux from the magnet extends through the detection surface, and a magnetic path is formed between the detection surface 25 and the ring gear 21. The ring gear 21 forms a part of the magnetic path. When the teeth 26 pass by the detection surface 25 with the rotation of the ring gear 21, the magnetic resistance and the magnetic flux of the magnetic path between the detection surface 25 and the ring gear 21 change. The magnetic detection element outputs signals in accordance with the change of the magnetic flux. Accordingly, the teeth 26, which pass by the detection surface, are detected based on the signals.
The sensitivity of the magnetic sensor 22 is greatly influenced by the space between the tooth edge 26a and the detection surface 25. Accordingly, the magnetic sensor 22 is installed in the outer case 23 with the detection surface 25 as near as possible to the tooth edge 26a, taking into consideration the error of measurement of the teeth edges and the error of the installation position of the magnetic sensor 22 in the axial direction.
During assembly of the differential apparatus 20, the position of the inner case 27 in the axial direction is set to adjust the backlash between the ring gear 21 and the drive pinion. When the axial position of the inner case 27 changes, the space between the detection surface 25 of the magnetic sensor and the tooth edge 26a of the ring gear 21 changes. Since the position of the inner case 27 is adjusted in each individual differential apparatus 20, the space between the detection surface 25 and the tooth edge 26a differs from one differential apparatus 20 to another. Accordingly, the sensitivity of the magnetic sensor 22 differs in each differential apparatus 20. Also, when the space between the detection surface 25 and the tooth edge 26a is too large, the magnetic sensor 22 will not be able to detect gear teeth.
Thus, to keep the space between the detection surface and the tooth edge 26a constant, it was necessary to adjust the assembly position of the magnetic sensor 22 in the axial direction, for example, by arranging a shim (not shown) between the positioning flange 22a and the positioning surface 24a of the hole 24. This complicates the installation of the magnetic sensor 22. Even if the differential apparatus 20 assembled in this way is installed in a vehicle, the axial position of the inner case 27 can change when the rotation direction of the drive shaft is changed. Therefore, fluctuation of the sensitivity of the magnetic sensor 22 was not completely prevented by using a shim.